Biological self assembly and biomolecular recognition continue to inspire novel approaches for the fabrication of bioanalytical, electrical and optical devices, via the organization of building blocks into hierarchical structures (Mann, S., Biomimetic materials chemistry. VCH: New York, 1996; p xvi, 383 p.; Weiner, S.; Addadi, L.; Wagner, H. D., Materials design in biology. Materials Science & Engineering C-Biomimetic and Supramolecular Systems 2000, 11 (1), 1-8). Self-assembling peptide and protein systems have been used to make wires, fibers, tubes, vesicles and other structures that are highly ordered on the nanometer scale (Zhang, S. G., Fabrication of novel biomaterials through molecular self-assembly. Nat Biotechnol 2003, 21 (10), 1171-1178; Hartgerink, J. D.; Beniash, E.; Stupp, S. I., Self-assembly and mineralization of peptide-amphiphile nanofibers. Science 2001, 294 (5547), 1684-1688; Aggeli, A.; Nyrkova, I. A.; Bell, M.; Harding, R.; Carrick, L.; McLeish, T. C. B.; Semenov, A. N.; Boden, N., Hierarchical self-assembly of chiral rod-like molecules as a model for peptide beta-sheet tapes, ribbons, fibrils, and fibers. Proc. Natl. Acad. Sci. USA 2001, 98 (21), 11857-11862). However, the potential of these systems for assembling devices is limited in part by difficulties in controlling the geometry of the self-assembling components. A significant prerequisite for the fabrication of useful nanodevices in electronic and biomedical applications is the construction of well-ordered two-dimensional structures, because most device architectures are based on the stacking of two-dimensional components. Sheet-like two-dimensional materials are a fundamentally important geometry for device construction, and yet there are relatively few ways to prepare such materials by self-assembly. Their use includes components of bioanalytical devices, electrochemical devices (such as batteries, fuel cells and supercapacitors), membranes for filtration and separation, surface coatings with chemically defined surfaces and biosensors.
Currently, two-dimensional assembly has been achieved at the interface of liquid-liquid, liquid-solid and liquid-air which acts as a template. Two popular methods are self assembled monolayers (SAM) and Langmuir-Blodgett monolayers (LBM) (Ulman, A., Formation and structure of self-assembled monolayers. Chemical Reviews 1996, 96 (4), 1533-1554). The self-assembled monolayers are based on the covalent bonding between the assembling molecules and a surface template (Bain, C. D.; Whitesides, G. M., Molecular-Level Control over Surface Order in Self-Assembled Monolayer Films of Thiols on Gold. Science 1988, 240 (4848), 62-63). The template is an absolute requirement, as the monolayers are chemically bonded to the surface. In the Langmuir-Blodgett monolayers, the hydrophobic molecules are segregated and spread to form a monolayer at the air-water interface. The assembled species can be transferred to the other processable solid surfaces, but this is a delicate process as the films are often quite fragile, and the process is not scaleable.
Chemical synthesis also represents another research direction to fabricate two-dimensional nanostructures. A variety of inorganic materials have been fabricated into planar structures of various sizes using solution-based methods (Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S., Preparation and characterization of graphene oxide paper. Nature 2007, 448 (7152), 457-460; Viculis, L. M.; Mack, J. J.; Kaner, R. B., A chemical route to carbon nanoscrolls. Science 2003, 299 (5611), 1361-1361; Tang, Z. Y.; Zhang, Z. L.; Wang, Y.; Glotzer, S. C.; Kotov, N. A., Self-assembly of CdTe nanocrystals into free-floating sheets. Science 2006, 314 (5797), 274-278). In these cases, electrostatic interactions and anisotropic hydrophobic attraction were used to drive the assembly of nanocrystals into sheet structures. However, the range of composition is limited to certain kinds of oxide materials which have the layered structure in one direction. Additionally, it is hard to generalize the strategy and can be rarely extended to other sheets with different compositions.
Biomolecules such as proteins (Xu, G. F.; Wang, W. X.; Groves, J. T.; Hecht, M. H., Self-assembled monolayers from a designed combinatorial library of de novo beta-sheet proteins. Proc. Natl. Acad. Sci. USA 2001, 98 (7), 3652-3657), peptides (Rapaport, H.; Kjaer, K.; Jensen, T. R.; Leiserowitz, L.; Tirrell, D. A., Two-dimensional order in beta-sheet peptide monolayers. Journal of the American Chemical Society 2000, 122 (50), 12523-12529; Zhang, S. G.; Holmes, T.; Lockshin, C.; Rich, A., Spontaneous Assembly of a Self-Complementary Oligopeptide to Form a Stable Macroscopic Membrane. Proc. Natl. Acad. Sci. USA 1993, 90 (8), 3334-3338; Vauthey, S.; Santoso, S.; Gong, H. Y.; Watson, N.; Zhang, S. G., Molecular self-assembly of surfactant-like peptides to form nanotubes and nanovesicles. Proc. Natl. Acad. Sci. USA 2002, 99 (8), 5355-5360; Aggeli, A.; Bell, M.; Boden, N.; Keen, J. N.; Knowles, P. F.; McLeish, T. C. B.; Pitkeathly, M.; Radford, S. E., Responsive gels formed by the spontaneous self-assembly of peptides into polymeric beta-sheet tapes. Nature 1997, 386 (6622), 259-262), lipids (Groves, J. T.; Ulman, N.; Boxer, S. G., Micropatterning fluid lipid bilayers on solid supports. Science 1997, 275 (5300), 651-653), and DNA (Winfree, E.; Liu, F. R.; Wenzler, L. A.; Seeman, N. C., Design and self-assembly of two-dimensional DNA crystals. Nature 1998, 394 (6693), 539-544) have also been demonstrated to assemble into two-dimensional structures. These materials may suffer many disadvantages as a material to fabricate into devices. They are typically not stable for long periods, or at extremes of temperature, pH, solvents or other non-physiological conditions. They can be difficult to produce on a large scale. They can rarely from stable, flat two-dimensional sheet structures over long distance scales. This hinders innovation and inhibits realization of commercial applications of biological self-assembly. One challenge with achieving highly ordered self assembled peptide-based materials is that the high amount of hydrogen bonding can lead to kinetically trapped structures, making it difficult to form a thermodynamically stable uniform structure.